I always create an existential relation between myself and the "I" of the Cartesian "I think, I exist" whenever, and while, I am performing the thought-act "I think. I exist" in the first person, present tense mode. Furthermore, my "I think, I exist" performance is always existentially consistent and, therefore, existentially self-verifying. And whenever I try to perform its negation "I am not thinking, I am not existing" in the first person, present tense mode, it always turns out to be existentially inconsistent and, therefore, existentially self-defeating, i.e., impossible.
By contrast, I cannot create an existential relation between myself and the I of the Cartesian inferential proposition "I think, therefore I exist" because the inferential proposition provides nothing more than an objective written version or representation of my original subjective performance or thought-act. And because I cannot create an existential relation between myself and the I of the inferential proposition, the fundamental notions of existential consistency and existential self-verification are neither applicable, nor relevant, to the inferential proposition. Only the notion of logical validity is applicable and relevant to the inferential proposition.
The Cogito Sum inferential proposition can be interpreted to be a derivative rendition of my original Cogito Sum performance because the objective logical truths expressed in writing by the Cogito Sum proposition are ultimately dependent upon and derived from the more primordial existentially consistent and existentially self-verifying truths that result from my subjective Cogito Sum performance.
It is the existentially consistent and existentially self-verifying truths resulting from my Cogito Sum performance which provide the necessary and sufficient ontological pre-conditions that support the existence of the objective logical truths that are expressed in written form by the Cogito Sum inferential proposition.
Because the existence of the Cogito Sum inferential proposition is dependent on the Cogito Sum performance, this explains why modus ponens is applicable to the Cogito Sum and why the Cogito Sum appears to involve both a valid inference regarding the truth of my existence and a more primordial performance-based existentially consistent and existentially self-verifying intuition of the truth of my existence.
I submit that this is precisely what Descartes meant when he stated, "When someone says 'I am thinking, therefore I am, or I exist,' he does not deduce existence from thought by means of a syllogism, but recognizes it as something self-evident by a simple intuition of the mind."
So, for the above reasons, I submit that if someone is going to attempt to really critique the truth of the Cogito, they should focus their efforts on the Cogito Sum as a performance, not on the Cogito, ergo Sum as an inferential proposition.
A serious critic of the indubitable certainty of the truth of the Cogito Sum performance, who hoped to be successful, would have to come up with a hyperbolic doubt that can neutralize or nullify the existential consistency and existential self-verification of the truth of the Cogito Sum performance when and while it is occurring in the mind of the meditator in the first person, present tense mode.